The broader goal of this application is to understand the mechanisms that regulate translation of the Cdk inhibitor p27Kip2 (p27) in normal cells and in breast cancer cells. p27 is an essential component of normal cell cycle control and is expressed at elevated levels in quiescent cells and down-regulated of normal cell cycle control and is expressed at elevated levels in quiescent cells and down-regulated upon reentry into the cell cycle. There is a very strong correlation between low p27 expression and poor clinical prognosis for numerous types of cancer, including breast cancer. p27 levels are regulated by translation initiation efficiency and by protein stability. This proposal focuses specifically on translational control of p27. Evidence is provided demonstrating that the p27 mRNA can be translated by a cap-independent mechanism through an internal ribosome entry site. Based on these findings,, a model has been developed for p27 translational control. In order to test this model three specific aims are proposed. The third specific aim is to determine if changes in p27 translation that occur during cell proliferation and differentiation are mediated through the internal ribosome entry site and to determine if activity of eIF-4E, the cap-binding protein, influences this process. The second aim is to determine if the decrease in 27 expression that is observed in breast cancer cells involves changes that affect p27 translation through the internal ribosome entry site. Since eIF-4E is over- expressed in virtually all breast cancer cells, its potential role in low p27 expression will be examined. The final aim is to characterize the molecular components required for internal ribosome assembly on the p27 mRNA. This will include identification of the RNA motifs involved, characterization of RNA-protein interactions, purification of the factors involved, and cloning of their cDNAs.